This project is concerned with the structure and organization of perceived representations, how such structure changes with the developmental level of the subjects, and how different perceived structures limit the subsequent processing options of the subject. The basic hypothesis is that the structure of the representation and the possible processing options depend upon 1) the specific combination of multidimensional stimuli, and 2) the age of the subject. Integral combinations (e.g., hue and brightness) yield perceptually wholistic stimuli that are characterized by a similarity structure and do not allow selective attention. In contrast, separable combinations (e.g., size of circle and angle of radial line) yield stimuli that are perceived dimensionally and do allow selective attention. According to the separability hypothesis, dimensional combinations that are perceived by the adult as separable are perceived by the young child as integral. The aims of our research are 1) to determine the nature of perceived structure and its development, 2) to specify the kinds of learning experiences that are important in producing changes in the structure of perceptual representation, 3) to evaluate the development of attention in relation to the development of different modes of perceptual organization. The approach that is taken to this research uses different tasks (similarity judgment, classification, reaction time) to provide converging operations for specific conceptualizations. The principal independent variables include dimensional combination, stimulus factors, structure of stimulus sets, encoding time, and age.